1. Field of the Invention
The present invention pertains to seismic data processing, and, more particularly, to a technique for handling static corrections in multiple prediction.
2. Description of the Related Art
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying typically utilizes seismic energy sources and seismic receivers located in the water which are either towed behind a vessel or positioned on the water bottom from a vessel. The energy source is typically an explosive device or compressed air system which generates acoustic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves reflects back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded.
Seismic waves, however, reflect from interfaces other than just those between subterranean formations, as would be desired. Seismic waves also reflect from the water bottom and the water surface, and the resulting reflected waves themselves continue to reflect. Waves which reflect multiple times are called “multiples”. Waves which reflect multiple times in the water layer between the water surface above and the water bottom below are called “water-bottom multiples”. Water-bottom multiples have long been recognized as a problem in marine seismic processing and interpretation, so multiple attenuation methods based on the wave equation have been developed to handle water-bottom multiples. However, a larger set of multiples containing water-bottom multiples as a subset can be defined. The larger set includes multiples with upward reflections from interfaces between subterranean formations in addition to upward reflections from the water bottom. The multiples in the larger set have in common their downward reflections at the water surface and thus are called “surface multiples”.
One difficulty with multiples is the effect of environmental anomalies addressed by static corrections. Often called “statics,” a static correction is a bulk shift of a seismic trace in time during seismic processing to “correct” for the effects of environmental anomalies. Generally, statics are caused by structural or velocity variations in the near surface. These variations may be spatial (e.g., elevation statics) or temporal (e.g., tidal statics). In land-based surveys, common static corrections compensate for a layer of low seismic velocity material near the surface of the Earth, differences in topography, and differences in the elevations of sources and receivers. In marine surveys, common static corrections compensate for changes in tidal conditions and water velocity.
Historically, seismic surveys were conducted in two-dimensions, i.e., they were two-dimensional (“2D”). In 2D survey data processing, spatial statics may be problematical, depending on the processing technique, but temporal statics are generally not problematical because the effect of the static varies slowly with spatial location. However, seismic surveys are now increasingly being conducted in three dimensions, i.e., they are three-dimensional (“3D”). In 3D surveys, statics are a more significant issue. Statics have been commonly applied to primaries in 3D surveys, but not to multiples. For primaries, the static is usually a simple time shift. The effect of statics on multiples is fundamentally different to that for primaries. Consider the case of an nth order surface multiple in a surface marine survey. The raypath for this multiple passes through the region creating the static anomaly (n+1) times as often as the corresponding primary, and therefore the required static correction is of the order of (n+1) times as large. The application of statics derived for the primaries will therefore not completely correct the static problems for the multiples.
A large class of multiple prediction algorithms, (e.g., 2D SRME, 3D SRME and WEMA) predict multiples from primaries and lower-order multiples. These algorithms assume that the data are consistent with a single model, at least within each subset of the data that are processed together. When this assumption is not met because of variations in the statics, static corrections must be applied in order to make the primaries consistent. This is likely to be the case for 3D algorithms, where data from different sail lines are processed together. However, simply correcting the data for the primary statics does not solve the problem, because the statics for the multiples are different.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.